By having a set of platform sprites that are all a regular size (128 x 128), it is straightforward to create a Tile Palette from those sprites, and then to add a Grid and Tilemap to the scene, allowing the Tile Palette brush to paint tiles into the scene. By doing this, we added platforms to this scene that are all well-aligned with each other, both horizontally and vertically.
You had to set the Sprite pixels per unit to 128, matching the size of these sprites, so that each Tile maps to a 1 x 1 Unity Grid unit. If we were to use different size sprites (say, 256 x 256), then the pixels per unit must be set to that size, again to achieve a 1 x 1 Unity Grid.
You added a Tilemap Collider 2D to the Tilemap GameObject so that characters (such as the potatoman) can interact with the platforms. Without a Collider 2D, these tiles would have seemed just part of the background graphics. By adding a Layer Ground and setting the Tilemap GameObject to this Layer...
